Semiconductor device in which zinc oxide is used as a semiconductor material and method for manufacturing the semiconductor device

ABSTRACT

A semiconductor device having excellent crystallinity and excellent electric characteristics includes a ZnO thin film having excellent surface smoothness. ZnO-based thin films (an n-type contact layer, an n-type clad layer, an active layer, a p-type clad layer, and a p-type contact layer) primarily including ZnO are formed sequentially by an ECR sputtering method or other suitable method on a zinc-polar surface of a ZnO substrate. A transparent electrode and a p-side electrode are formed by an evaporation method or other suitable method on a surface of the p-type contact layer, and an n-side electrode is formed on an oxygen-polar surface of the ZnO substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device. In particular, the presentinvention relates to a semiconductor device in which zinc oxide is usedas a semiconductor material and to a method for manufacturing thesemiconductor device.

2. Description of the Related Art

Zinc oxide (ZnO) is one type of II-VI compound semiconductor. The bandgap energy of ZnO can be changed by making ZnO into a mixed crystal withMgO, CdO, or other suitable materials, and ZnO can have a multilayerstructure of quantum well and other suitable structures. Furthermore,since the bond energy of an exciton is very large, ZnO is suitable foruse in light-emitting devices. Since ZnO is transparent in the visiblerange, ZnO is also suitable for use in transparent thin film transistorsfor driving liquid crystal displays.

Meanwhile, ZnO has a wurtzite structure (hexagonal system). As shown inFIG. 9, ZnO has no center of symmetry in the c axis direction (verticaldirection) and has polarity based on a molecular structure.

That is, ZnO has zinc-polarity (+c polarity) in which three bondsbonding to a Zn atom 51 point downward and three bonds bonding to anoxygen atom 52 point upward, as shown in FIG. 9( a), and oxygen-polarity(−c polarity) in which three bonds bonding to a Zn atom 51 point upwardand three bonds bonding to an oxygen atom 52 point downward, as shown inFIG. 9( b).

Here, the above-described polarity refers to an orientation of theabove-described bond and does not refer to an element terminating thesurface.

It has been reported that a ZnO thin film having the oxygen-polarity waspreviously formed by a PMBE (plasma-assisted molecular-beam epitaxy)method on a sapphire substrate (APPLIED PHYSICS LETTERS, Vol. 80, No. 8,pp. 1358-1360 (2002); hereafter referred to as “first knowntechnology”).

It has been reported that a film of GaN having Ga-polarity was formed ona sapphire substrate and, by controlling the film formation conditions,a ZnO thin film having the zinc-polarity or the oxygen-polarity wasformed on the above-described GaN (APPLIED PHYSICS LETTERS, Vol. 77, No.22, pp. 3571-3573 (2000); hereafter referred to as “second knowntechnology”).

In addition, another technology has been reported, in which the polarityof a piezoelectric film of ZnO formed on a substrate was able to bespecified (Japanese Unexamined Patent Application Publication No.2001-144328; hereafter referred to as “third known technology”), asanother known technology.

In the above-described third known technology, a piezoelectric film (ZnOfilm) having a + surface (zinc-polarity) or a − surface(oxygen-polarity) can be formed in accordance with the type ofsubstrate, and the polarity of the piezoelectric film of ZnO formed on asubstrate is controlled by changing the film formation conditions, e.g.,a heating temperature of the substrate.

With respect to the above-described first known technology, it has beendetermined by Coaxial Impact Collision Ion Scattering Spectroscopy(CAICISS) that the ZnO thin film formed on the sapphire substrate hasoxygen-polarity. However, substantially hexagonal crystal grains remainin such a ZnO film, the surface shape becomes uneven and, thereby, thedesired surface smoothness of the ZnO thin film cannot be obtained.

That is, since the ZnO thin film formed by the first known technologyhas poor surface smoothness, where a semiconductor device is formedusing the ZnO thin film, a current passes through grain boundaries, anda concentration of electric field occurs on convex portions of crystalgrains. Consequently, the operation of the device may become unstable,or the device may be destroyed.

According to the second known technology, the polarity of the ZnO thinfilm can be controlled by changing the film formation conditions. Inthis manner, the ZnO thin film having the zinc-polarity or theoxygen-polarity can be formed on GaN. However, the substrate temperatureincreases during the film formation of ZnO on GaN and, thereby, Ga,which is an element of GaN, may diffuse into the ZnO thin film.

Since Ga functions as a donor to ZnO, if Ga diffuses into the ZnO thinfilm, the resistance of ZnO is reduced.

Furthermore, it is difficult to control the above-described diffusionand, therefore, variations may occur in the device characteristics ofthe semiconductor device.

In the above-described second known technology, since there is latticemismatch between GaN and ZnO, lattice defects are introduced to mitigatethe lattice mismatch. As a result, the crystallinity of the ZnO thinfilm is deteriorated and, thereby, deterioration of the electriccharacteristics occurs.

The above-described third known technology discloses the formation ofthe piezoelectric thin film having the zinc-polarity or theoxygen-polarity. However, there is no disclosure with respect to theinfluence exerted by the polarity on the surface shape and the electriccharacteristics of the thin film. Furthermore, since the material forthe substrate is different from the material for the piezoelectric film,deterioration of the crystallinity may occur due to the lattice mismatchas in the second known technology, and there is a problem in that highlyreliable, desirable, and excellent electric characteristics cannot beobtained.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a semiconductor device, which is provided witha ZnO thin film having excellent surface smoothness and which hasexcellent crystallinity and excellent electric characteristics, and amethod for manufacturing the semiconductor device.

The inventors of the present invention conducted intensive research inorder to obtain a ZnO thin film having excellent surface smoothness. Asa result, it was discovered that a semiconductor device having excellentsurface smoothness and crystallinity and excellent electriccharacteristics could be obtained by forming a ZnO thin film on azinc-polar surface of a single crystal substrate, primarily includingzinc oxide.

A semiconductor device according to a preferred embodiment of thepresent invention includes a single crystal substrate primarilyincluding zinc oxide which has a zinc-polar surface and an oxygen-polarsurface and at least one layer of thin film primarily including zincoxide is disposed on the above-described zinc polar surface. In thismanner, a thin film having excellent surface smoothness andcrystallinity is produced.

The inventors of the present invention examined the polarity of theabove-described thin film, and it was confirmed that the thin film hadzinc-polarity.

Therefore, the semiconductor device according to a preferred embodimentof the present invention includes the above-described thin film havingthe zinc-polarity. That is, a ZnO-based thin film formed on a zinc-polarsurface of a ZnO substrate has the zinc-polarity.

In the semiconductor device according to preferred embodiments of thepresent invention, the above-described thin film is composed of amultilayer film, and the multilayer film defines a light-emitting layeror a switching portion.

Specifically, the above-described thin film is composed of themultilayer film, the multilayer film defines the light-emitting layerand, therefore, the light-emitting layer has excellent surfacesmoothness and crystallinity. Consequently, a light-emitting device,e.g., LED and LD, having excellent electric characteristics is obtained.

Alternatively, the above-described thin film is composed of themultilayer film, the multilayer film defines the switching portion and,therefore, even when light is applied to an active layer, changes in theelectrical conductivity are minimized. Consequently, semiconductordevices, e.g., TFT, capable of preventing deterioration of thecharacteristics of the switching element are readily obtained.

Since the above-described semiconductor device is provided with aZnO-based multilayer film having excellent surface smoothness andcrystallinity, semiconductor devices, e.g., light-emitting elements andthin film transistors, having excellent electric characteristics arereadily obtained.

A method for manufacturing a semiconductor device according to preferredembodiments of the present invention includes the steps of determiningwhether a surface of a single crystal substrate primarily including zincoxide is a zinc-polar surface or an oxygen-polar surface, and forming atleast one layer of thin film primarily including zinc oxide on theabove-described zinc-polar surface.

According to the above-described manufacturing method, a desiredZnO-based thin film is readily and reliably formed on the zinc-polarsurface of the zinc oxide substrate.

That is, after determining whether the surface of the ZnO substrate is azinc-polar surface or an oxygen-polar surface, at least one layer ofthin film primarily including ZnO is formed on the above-describedzinc-polar surface. Since the above-described ZnO thin film haszinc-polarity, the ZnO-based thin film is readily formed on thezinc-polar surface.

The semiconductor device according to preferred embodiments of thepresent invention is preferably formed using a sputtering apparatusprovided with a plasma generation chamber and a film formation chamber.A sputtering treatment is performed using the sputtering apparatus so asto form the above-described thin film.

According to the above-described manufacturing method, since the film isformed by the sputtering treatment, a semiconductor device havingdesired electric characteristics is obtained inexpensively. Furthermore,since the plasma generation chamber and the film formation chamber areseparated, plasma damage to the semiconductor device is minimized.

Preferably, the above-described sputtering treatment is performed by anymethod selected from an electron cyclotron resonance plasma sputteringmethod, an inductively coupled plasma sputtering method, a helicon waveexcited plasma sputtering method, an ion beam sputtering method, and acluster beam sputtering method. Alternatively, the above-described thinfilm is performed preferably by any method selected from among amolecular-beam epitaxy method, a metal organic chemical vapor depositionmethod, a laser molecular-beam epitaxy method, and a laser abrasionmethod.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a first preferred embodiment ofa semiconductor device according to the present invention.

FIGS. 2A and 2B are diagrams showing polar characteristics of ZnO.

FIG. 3 is a sectional view schematically showing the surface shape of aZnO thin film formed on a zinc-polar surface of a ZnO substrate.

FIG. 4 is a diagram showing polar characteristics of a ZnO thin filmformed on the zinc-polar surface of the ZnO substrate.

FIG. 5 is a schematic sectional view of a second preferred embodiment ofthe semiconductor device according to the present invention.

FIG. 6 is a schematic sectional view of a preferred third embodiment ofthe semiconductor device according to the present invention.

FIG. 7 is a micrograph showing the surface shape of a ZnO thin filmformed on a zinc-polar surface of a ZnO substrate.

FIG. 8 is a micrograph showing the surface shape of a ZnO thin filmformed on an oxygen-polar surface of the ZnO substrate.

FIGS. 9A and 9B are diagrams showing a crystal structure of ZnO.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowin detail with reference to the drawings.

FIG. 1 is a schematic sectional view of a Light Emitting Diode(hereafter referred to as “LED”) as a first preferred embodiment of asemiconductor device according to the present invention.

In FIG. 1, reference numeral 1 denotes a single crystal substrateprimarily including electrically conductive ZnO having an n-typeconduction form (hereafter referred to as “ZnO substrate”), and the ZnOsubstrate 1 has a zinc-polar surface 1 a and an oxygen-polar surface 1b.

In the LED, a light-emitting layer 2 is disposed on the zinc-polarsurface 1 a of the ZnO substrate 1, and a transparent electrode 3 madeof Indium Tin Oxide (hereafter referred to as “ITO”) and having a filmthickness of about 150 nm is disposed on the surface of thelight-emitting layer 2. Furthermore, a p-side electrode 4 having a totalfilm thickness of about 300 nm is disposed on a substantially centralportion of the surface of the transparent electrode 3, while a Ni film,an Al film, and a Au film are laminated sequentially in the p-sideelectrode 4.

An n-side electrode 5 having a total film thickness of about 300 nm isdisposed on the oxygen-polar surface 1 b of the ZnO substrate 1, while aTi film and a Au film are laminated sequentially in the n-side electrode5.

Specifically, the above-described light-emitting layer 2 includes amultilayer film in which an n-type contact layer 6, an n-type clad layer7, an active layer 8, a p-type clad layer 9, and a p-type contact layer10 are laminated sequentially. That is, the active layer 8 is disposedbetween the n-type clad layer 7 and the p-type clad layer 9, the n-typeclad layer 7 is connected to the n-side electrode 5 with the n-typecontact layer 6 and the ZnO substrate 1 therebetween, and the p-typeclad layer 9 is connected to the transparent electrode 3 with the p-typecontact layer 10 therebetween.

The active layer 8 is formed from Cd_(x)Zn_(1-x)O (where x satisfies0≦x<1, and is about 0.1, for example) which is a mixed crystal of CdOand ZnO and which has a film thickness of about 200 nm.

The active layer 8 emits light by recombination of an electron which isan n-type carrier and a hole which is a p-type carrier, and thewavelength of the emitted light is determined by the band gap energy.

Since carriers must be effectively confined in the active layer 8, then-type clad layer 7 and the p-type clad layer 9 have a band gap energythat is greater than that of the above-described active layer 8, and arecomposed of Mg_(y)Zn_(1-y)O (where y satisfies 0≦y<1, and is about 0.2,for example) which is a mixed crystal of MgO and ZnO. The n-type cladlayer 7 has a film thickness of about 2,000 nm, and the p-type cladlayer 9 has a film thickness of about 600 nm.

Both the n-type contact layer 6 and the p-type contact layer 10 areformed from ZnO and have a film thickness of about 200 nm.

A method for manufacturing the above-described LED will be describedbelow.

Initially, a ZnO single crystal is prepared by a SCVT (Seeded ChemicalVapor Transport) method or other suitable method. A surfaceperpendicular to the c axis direction of the crystal axis is cut fromthe ZnO single crystal and is subjected to mirror polishing, such that aZnO substrate is prepared and the polarity thereof is checked.

Examples of known methods for determining the polarity of a compoundsemiconductor, e.g., ZnO, having a piezoelectric property include aCoaxial Impact Collision Ion Scattering Spectroscopy (CAICISS) method(APPLIED PHYSICS LETTERS, Vol. 72, (1998), p. 824), a Convergent BeamElectron Diffraction (CBED) method (APPLIED PHYSICS LETTERS, Vol. 69,(1996), p. 337), and a Scanning Nonlinear Dielectric Microscopy (SNDM)method (Sentangijutsu Symposium, “Atsudenzairyoto Danseiha Device,”(High Technology Symposium, “Piezoelectric Material and Elastic WaveDevice”), (February, 2000) pp. 23-30). In the present preferredembodiment, the polarity of the ZnO substrate is checked by the SNDM(Scanning Nonlinear Dielectric Microscopy) method.

That is, in the SNDM, when a potential is applied while a probe scansthe ZnO substrate 1, an intensity signal incorporating the polarity ofthe ZnO substrate 1 is detected.

On the other hand, when the applied potential is “0”, since thepotential is not applied, no intensity signal incorporating the polarityis detected.

In the SNDM method, when a potential is applied to the ZnO substrate 1,the intensity signal is displaced to the + side or the − side ascompared to when the applied potential is “0”.

Therefore, in the SNDM method, the intensity at an applied potential of“0” is taken as the reference signal, and the intensity signal when apotential is applied while the probe scans the ZnO substrate 1 is takenas the polarity signal. Then, the polarity of the ZnO substrate isdetermined based on the displacement of the polarity signal toward the +side or the − side relative to the reference signal.

In the present preferred embodiment, the displacement of the polaritysignal toward the − side relative to the reference signal indicatesthe + polarity (zinc-polarity), and the displacement of the polaritysignal toward the + side relative to the reference signal indicates the− polarity (oxygen-polarity) based on the configuration of the SNDM.

FIGS. 2( a) and 2(b) are diagrams showing the polar characteristics ofthe ZnO substrate 1. The horizontal axis indicates the scanning length(μm), and the vertical axis indicates the intensity (a.u. indicatesarbitrary unit).

In FIGS. 2( a) and 2(b), the direction indicated by the arrow Xrepresents the polarity signal of the ZnO substrate 1, and the directionindicated by the arrow X′ represents the reference signal when nopotential is applied.

Therefore, when the polarity signal is displaced toward the − siderelative to the reference signal as shown in FIG. 2( a), the polarsurface of the ZnO substrate 1 is a zinc-polar surface, and when thepolarity signal is displaced toward the + side relative to the referencesignal as shown in FIG. 2( b), the polar surface of the ZnO substrate 1is an oxygen-polar surface.

After the polarity of the ZnO substrate 1 is determined as describedabove, a ZnO thin film is laminated on the zinc-polar surface 1 a of theZnO substrate 1 by an Electron Cyclotron Resonance (hereafter referredto as “ECR”) sputtering apparatus.

That is, the ECR sputtering apparatus having a plasma generation chamberand a separate film formation chamber is provided, the ZnO substrate 1is disposed at a predetermined position in the film formation chamberwith the zinc-polar surface 1 a facing up, and the ZnO substrate 1 isheated to a temperature of about 300° C. to about 800° C.

Subsequently, a reactive gas, e.g., oxygen, and a plasma generation gas,e.g., argon, are supplied to the plasma generation chamber, and amicrowave is discharged at a frequency (e.g., about 2.45 GHz) at whichresonance occurs in the cyclotron, such that plasma is generated in theplasma generation chamber.

Thereafter, a high-frequency power (for example, about 150 W) is appliedto a sputtering target, and a sputtering target substance (ZnO) issputtered using the plasma generated in the plasma generation chamber,such that the n-type contact layer 6 made of ZnO is formed on thesurface of the ZnO substrate 1 by reactive sputtering.

Next, the reactive sputtering is performed using a target produced bysintering MgO and ZnO at a desired mixing ratio, such that the n-typeclad layer 7 made of Mg_(y)Zn_(1-y)O (where 0≦y<1) is formed.

Likewise, the reactive sputtering is performed, and the active layer 8made of Cd_(x)Zn_(1-x)O (where 0≦x<1), the p-type clad layer 9 made ofMg_(y)Zn_(1-y)O (where 0≦y<1), and the p-type contact layer 10 made ofZnO are formed sequentially.

The film thickness of each thin film is set at a desired film thicknessby controlling the reaction time.

The Ti film and the Au film are formed sequentially on the surface ofthe oxygen-polar surface 1 b of the ZnO substrate 1 so as to form then-side electrode 5 by an evaporation method, the ITO film is formed onthe surface of the p-type contact layer 10 by the evaporation method soas to form the transparent electrode 3 and, thereafter, Ni, Al, and Auare laminated sequentially so as to form the p-side electrode 4.

As described above, in the first preferred embodiment, thelight-emitting layer 2 composed of a ZnO-based multilayer film is formedon the zinc-polar surface 1 a of the ZnO substrate 1 and, thereby,ZnO-based thin films having excellent surface smoothness are laminatedsequentially. As a result, the surface of the n-type clad layer 7becomes a thin film including a smooth terrace 11 and a linear step 12,as shown in FIG. 3 (magnified diagram of a portion A shown in FIG. 1),and having excellent surface smoothness. Therefore, any crystal grainand any uneven portion are eliminated from the surface, and the ZnO thinfilm having excellent surface smoothness is obtained.

The thin film having excellent surface smoothness, as shown in FIG. 3,can be obtained with respect to not only the n-type clad layer 7, butalso the n-type contact layer 6, the active layer 8, the p-type cladlayer 9, and the p-type contact layer 10 in a similar manner.

Since the above-described ZnO-based thin films have excellent surfacesmoothness as described above, no current passes through grainboundaries, and a concentration of electric field on the surface of theZnO film does not occur. Consequently, no scattering occurs duringmovement of electrons, and an LED having high mobility of electron,excellent crystallinity, and excellent electric characteristics isobtained.

In the above-described first preferred embodiment, since the ECRsputtering apparatus is used and the ZnO-based thin films are formed bythe sputtering treatment, no expensive apparatus is required to beseparately provided, and thus, the thin film formation is performedinexpensively.

Furthermore, since the plasma generation chamber and the film formationchamber are separated, plasma damage to the ZnO thin film is minimized,and a thin film having good quality is obtained.

In the present preferred embodiment, the polarity of the ZnO thin filmformed on the surface of the ZnO substrate 1 was further examined by theuse of the SNDM.

That is, the sensitivity of the SNDM in the depth direction isdetermined based on the probe end radius of the probe and the dielectricconstant of the sample, ZnO. In the case of ZnO, the detectable range inthe depth direction is substantially equal to the probe end radius ofthe probe. Therefore, by making the above-described probe end radiussmaller than the film thickness, the polarity of the ZnO thin film canbe determined regardless of the polarity of the ZnO substrate 1 whichdefines the base.

FIG. 4 is a diagram showing the polar characteristics of the ZnO thinfilm. As in FIGS. 2( a) and 2(b), the horizontal axis indicates thescanning length (μm), the vertical axis indicates the intensity (a.u.),the direction indicated by an arrow X represents the polarity signal ofthe ZnO substrate 1, and the direction indicated by an arrow X′represents the reference signal when no potential is applied.

As shown in FIG. 4, since the polarity signal is displaced toward the −side relative to the reference signal, the ZnO thin film has thezinc-polarity. That is, the ZnO-based thin film formed on the zinc-polarsurface of the ZnO substrate 1 has the zinc-polarity.

In the present preferred embodiment, a double heterostructure is usedfor the light-emitting layer 2, in which the active layer 8 is disposedbetween the p-type clad layer 9 and the n-type clad layer 7. However, apn junction structure, an MIS (Metal-Insulating layer-Semiconductorlayer) structure, or a single heterostructure may be used.

FIG. 5 is a schematic sectional view of a Laser Diode (hereafterreferred to as “LD”) as a second preferred embodiment of thesemiconductor device according to the present invention.

In the LD, a light-emitting layer 14 is disposed on a zinc-polar surface13 a of a ZnO substrate 13 having electrical conductivity, and a p-sideelectrode 15 having a total film thickness of about 300 nm is disposedon the surface of the light-emitting layer 14, while a Ni film, an Alfilm, and a Au film are laminated sequentially in the p-side electrode15.

An n-side electrode 16 having a total film thickness of about 300 nm isdisposed on an oxygen-polar surface 13 b of the ZnO substrate 13, whilea Ti film and a Au film are laminated sequentially in the n-sideelectrode 16.

Specifically, the above-described light-emitting layer 14 is composed ofa multilayer film in which an n-type contact layer 17, an n-type cladlayer 18, an n-type light guide layer 19, an active layer 20, a p-typelight guide layer 21, a p-type clad layer 22, a current limiting layer23, and a p-type contact layer 24 are sequentially laminated.

That is, the active layer 20 is disposed between the n-type clad layer18 and the p-type clad layer 22 with the n-type guide layer 19 and thep-type guide layer 21 therebetween, respectively.

The n-type clad layer 18 is connected to the n-side electrode 16 withthe n-type contact layer 17 and the Zno substrate 13 therebetween, andthe p-type clad layer 22 is connected to the p-side electrode 15 withthe current limiting layer 23 and the p-type contact layer 24therebetween.

Specifically, the active layer 20 has a multi-quantum well structure inwhich 2 to 5 layers of a barrier layer composed of Mg_(y)Zn_(1-y)O(where y satisfies 0≦y<1, and is about 0.1, for example) and a welllayer composed of Cd_(x)Zn_(1-x)O (where x satisfies 0≦x<1, and is about0.1, for example), each having a thickness of about 3 nm, arealternately laminated.

When the refractive index of the active layer 20 is greater than thoseof the n-type clad layer 18 and the p-type clad layer 22, the light isconfined in the active layer 20. However, when the light is notadequately confined since the active layer 20 is a thin film, theleakage of the light from the active layer 20 must be prevented.Consequently, the n-type light guide layer 19 having a refractive indexbetween those of the n-type clad layer 18 and the active layer 20 isinterposed between the active layer 20 and the clad layer 18, and thep-type light guide layer 21 having a refractive index between those ofthe p-type clad layer 22 and the active layer 20 is interposed betweenthe active layer 20 and the clad layer 22 to define a portion of anoptical waveguide.

The n-type contact layer 17 having a film thickness of about 1,500 nmand made of ZnO is disposed on the zinc-polar surface 13 a of the ZnOsubstrate 13. The n-type clad layer 18 having a film thickness of about2,000 nm and made of Mg_(y)Zn_(1-y)O (where y satisfies 0≦y<1, and isabout 0.2, for example) is disposed on the surface of the n-type contactlayer 17. Furthermore, the n-type light guide layer 19 having a filmthickness of about 40 nm and made of ZnO is disposed on the surface ofthe n-type clad layer 18.

The active layer 20 having the above-described multi-well type structureis laminated on the surface of the n-type light guide layer 19. Thep-type light guide layer 21 having a film thickness of about 40 nm andmade of Mg_(y)Zn_(1-y)O (where y satisfies 0≦y<1, and is about 0.2, forexample) is disposed on the surface of the active layer 20. Furthermore,the p-type clad layer 22 having a film thickness of about 2,000 nm andmade of Mg_(y)Zn_(1-y)O (where y satisfies 0≦y<1, and is about 0.2, forexample) is disposed on the surface of the p-type light guide layer 21.

In addition, in order to pass a current only through an oscillationregion, the current limiting layer 23 having a film thickness of about400 nm and made of Mg_(y)Zn_(1-y)O (where y satisfies 0≦y<1, and isabout 0.2, for example) is disposed on the surface of the p-type cladlayer 22 and has a predetermined shape having a groove portion 23 a. Thep-type contact layer 24 is disposed on the surface of the p-type cladlayer 22 so as to have a cross section in the shape of a letter T whilecovering the current limiting layer 23.

The above-described LD is also produced by a method and proceduresubstantially similar to that in the first preferred embodiment of thepresent invention.

That is, initially, a ZnO single crystal is prepared by the SCVT methodor other suitable method. A surface perpendicular to the c axisdirection of the crystal axis is cut from the ZnO single crystal and issubjected to mirror polishing, such that a ZnO substrate is prepared andthe polarity thereof is checked by the SNDM.

Thereafter, as in the first preferred embodiment, the ECR sputteringapparatus is prepared, the ZnO substrate 13 is disposed at apredetermined position in a film formation chamber with the zinc-polarsurface 13 a up, and the ZnO substrate 1 is heated to a temperature ofabout 300° C. to about 800° C.

Subsequently, the reactive gas, e.g., oxygen, and the plasma generationgas, e.g., argon, are supplied to the plasma generation chamber, and amicrowave is discharged, such that plasma is generated in the plasmageneration chamber. A sputtering target substance (ZnO) is sputtered,and the n-type contact layer 17 made of ZnO is formed on the surface ofthe ZnO substrate 13 by reactive sputtering.

Likewise, the reactive sputtering is performed while the targetsubstance is appropriately changed to a desired substance, and then-type contact layer 17, the n-type clad layer 18, the n-type lightguide layer 19, the active layer 20, the n-type light guide layer 21,the p-type clad layer 22, and the current limiting layer 23 aresequentially formed.

After the current limiting layer 23 is formed, the resulting ZnOsubstrate 13 provided with the films is temporarily taken out of thesputtering apparatus. A photoresist is applied to the surface of theabove-described current limiting layer 23, the resist film is patternedby a known photolithographic technology, and an etching treatment isperformed with an alkaline solution, e.g., NaOH, such that the currentlimiting layer 23 is formed into a predetermined shape.

The above-described ZnO substrate 13 is again disposed at thepredetermined position in the ECR sputtering apparatus, and the reactivesputtering is performed, such that the film of p-type contact layer 24made of ZnO is formed to have a cross section in the shape of a letterT.

Thereafter, as in the first preferred embodiment, the Ti film and the Aufilm are sequentially formed on the surface of the oxygen-polar surface13 b of the ZnO substrate 13 so as to form the n-side electrode 16 by anevaporation method, and Ni, Al, and Au are sequentially laminated on thesurface of the p-type contact layer 24 by the evaporation method so asto form the p-side electrode 15.

As described above, in the second preferred embodiment, thelight-emitting layer 14 composed of a ZnO-based multilayer thin film isformed on the zinc-polar surface 13 a of the ZnO substrate 13 as in thefirst preferred embodiment. Therefore, ZnO-based thin films having asmooth terrace and a linear step are obtained. In this manner, sinceexcellent surface smoothness is provided, no current passes throughgrain boundaries, a concentration of electric field on the surface ofthe ZnO film does not occur. Consequently, no scattering occurs duringmovement of electrons, the mobility of electron becomes high, and thecrystallinity becomes excellent, such that an LD having excellentelectric characteristics is obtained.

In the above-described second preferred embodiment, as in the firstpreferred embodiment, since the ECR sputtering apparatus is used and theZnO-based thin films are formed by the sputtering treatment, noexpensive apparatus is required to be separately provided, and the thinfilm formation is performed inexpensively. Furthermore, since the plasmageneration chamber and the film formation chamber are separated, plasmadamage to the ZnO thin film is minimized, and a thin film having goodquality is obtained.

FIG. 6 is a schematic sectional view of a Thin Film Transistor(hereafter referred to as “TFT”) as a third preferred embodiment of thesemiconductor device according to the present invention. The TFTpreferably includes an insulating ZnO substrate 25, a gate electrode 26having a film thickness of about 50 nm and disposed on a substantiallycentral portion of the ZnO substrate 25, a gate insulating layer 27having a film thickness of about 200 nm and disposed on the ZnOsubstrate 25 while covering the gate electrode 26, an active layer 28having a film thickness of about 50 nm and disposed on the gateinsulating layer 27, a channel protective layer 29 having a filmthickness of about 200 nm and disposed on a substantially centralportion of the active layer 28, and a source electrode 30 and a drainelectrode 31 which are disposed so as to cover a portion of the channelprotective layer 29 and which have film thicknesses of about 200 nm.

In the above-described TFT, a switching portion includes constituentsother than the ZnO substrate 25, that is, the gate electrode 26, thegate insulating layer 27, the active layer 28, the channel protectivelayer 29, the source electrode 30, and the drain electrode 31. Theswitching portion defined by these elements is disposed on a zinc-polarsurface 25 a of the ZnO substrate 25.

The gate electrode 26, the source electrode 30, and the drain electrode31 have low resistances since ZnO is doped with Ga, and the gateinsulating layer 27 and the channel protective layer 29 have highresistances since ZnO is doped with Ni.

The active layer 28 is made of a non-doped ZnO thin film. The oxygenconcentration in the thin film is adjusted by controlling the oxygenpartial pressure during the formation of thin film and, thereby, theactive layer 28 is formed to have n-type conduction.

The above-described TFT can also be readily produced using the ECRsputtering and the photolithographic technology substantially similar tothat in the first and second preferred embodiments of the presentinvention.

That is, the ZnO substrate is prepared and, thereafter, the polarity isdetermined. Subsequently, the ECR sputtering apparatus is used, and thereactive sputtering is performed while Ga-doped ZnO is provided as atarget substance, such that a ZnO film (ZnO:Ga) is formed on azinc-polar surface 25 a of the ZnO substrate 25.

The resulting ZnO substrate 25 is removed from the ECR sputteringapparatus. A photoresist is applied to the above-described ZnO film, theresist film is patterned by a known photolithographic technology and,thereafter, an etching treatment is performed with an alkaline solution,e.g., NaOH, such that the gate electrode 26 is formed.

The reactive sputtering is performed while Ni-doped ZnO is provided as atarget substance, the gate insulating layer 27 is formed on the ZnOsubstrate 25 so as to cover the gate electrode. Subsequently, thereactive sputtering is performed while non-doped ZnO is provided as atarget substance and the oxygen partial pressure is controlled, suchthat the active layer 28 is formed.

Then, the reactive sputtering is performed while Ni-doped ZnO isprovided as a target substance, such that a ZnO film (ZnO:Ni film) isformed. A photoresist is applied to the resulting ZnO film as describedabove, the resist film is patterned by the photolithographic technologyand, thereafter, the etching treatment is performed with an alkalinesolution, e.g., NaOH, such that the channel protective layer 29 isformed.

Subsequently, the reactive sputtering is performed while Ga-doped ZnO isprovided as a target substance, a photoresist is applied to theresulting ZnO:Ga film as described above, the resist film is patternedby the photolithographic technology, and thereafter, the etchingtreatment is performed with an alkaline solution, e.g., NaOH, such thatthe source electrode 30 and the drain electrode 31 are formed.

As described above, in the third preferred embodiment, since the TFT isformed from a ZnO-based multilayer film, even when the active layer 28is exposed to light, a change in the electrical conductivity iseffectively suppressed.

That is, where an active layer is formed from amorphous silicon (a-Si),since a-Si becomes electrically conductive by the application of light,the characteristics of the switching element may be deteriorated. On theother hand, in the third preferred embodiment, since the active layer 28is formed from a ZnO thin film having a band gap of about 3.3 eV andhaving transparency to visible light, even when the light is applied tothe active layer 28, a change in the electrical conductivity isminimized, and deterioration of the characteristics of the switchingelement is prevented.

Furthermore, by integrally forming the TFT as an upper portion of aphotoelectric conversion element or the LED shown in the first preferredembodiment, the amount of light incident to the photoelectric conversionelement can be increased or the amount of light emitted from thelight-emitting layer can be increased and, therefore, the proportion ofopening is increased.

The present invention is not limited to the above-described preferredembodiments.

In the above-described preferred embodiments, the ZnO-based thin filmsare preferably formed by the ECR sputtering method. However, aninductively coupled plasma (ICP) sputtering method, a helicon waveexcited plasma (HWP) sputtering method, an ion beam sputtering method, acluster beam sputtering method, or other suitable sputtering method maybe used. Alternatively, the ZnO-based thin films may be formed by amolecular-beam epitaxy (MBE) method, a metal organic chemical vapordeposition (MOCVD) method, a laser molecular-beam epitaxy (laser MBE)method, a laser abrasion method, or other suitable method other than thesputtering method.

Specific examples of the present invention will be described below.

The inventors of the present invention used the ECR sputteringapparatus, formed a ZnO thin film on a zinc-polar surface of a ZnOsubstrate, and prepared a test piece for Example. Furthermore, a ZnOthin film was formed on an oxygen-polar surface of a ZnO substrate, anda test piece for Comparative example was prepared.

That is, an ECR sputtering apparatus having a plasma generation chamberand a separate film formation chamber was provided. A ZnO substrate wasdisposed at a predetermined position in the film formation chamber, andthe substrate was heated to a temperature of about 620° C.

Subsequently, each of 20 sccm of argon gas serving as a sputtering gasand 10 sccm of O₂ gas serving as a reaction gas was supplied to theplasma generation chamber, and micro-discharge was performed, such thatplasma was generated. A high-frequency electric field of about 150 W wasapplied to a sputtering target, and a sputtering treatment wasperformed, such that a Zno thin film was formed on a zinc-polar surfaceof a ZnO substrate, another ZnO thin film was formed on an oxygen-polarsurface of another ZnO substrate and, thereby, test pieces for theExample and the Comparative example were prepared.

The inventors of the present invention observed the surface shapes ofthe ZnO thin films with an atomic force microscope.

FIG. 7 shows the ZnO thin film of the Example. FIG. 8 shows the ZnO thinfilm of the Comparative example, formed on the oxygen-polar surface ofthe ZnO substrate.

As is clear from this FIG. 8, the ZnO thin film of the Comparativeexample is in the shape of islands and, therefore, grain boundaries arepresent.

On the other hand, as shown in FIG. 7, the ZnO thin film of the Examplehas a surface shape including a smooth terrace and a substantiallylinear step was obtained.

Therefore, it was clear that the ZnO thin film formed on the zinc-polarsurface of the ZnO substrate had significantly improved surfacesmoothness as compared to that of the ZnO thin film formed on theoxygen-polar surface of the ZnO substrate.

The inventors of the present invention calculated the root-mean-squaresurface roughness RMS of the ZnO thin film, and evaluated the surfaceroughness.

As a result, the root-mean-square surface roughness RMS of the ZnO thinfilm of the Comparative example was about 20.4 nm, whereas theroot-mean-square surface roughness RMS of the ZnO thin film of theExample was about 1.4 nm. Consequently, it was clear that the surfacesmoothness of the ZnO substrate was significantly improved by formingthe ZnO thin film on the zinc-polar surface as compared to that in thecase where the ZnO thin film was formed on the oxygen-polar surface.

The inventors of the present invention conducted a hole measurement, andcalculated the electron mobility.

When the crystallinity is excellent, the mobility is increased becauseelectrons are not scattered by crystal defects during movement. However,if crystal defects are present, the mobility is decreased becauseelectrons are scattered by the crystal defects during movement.

Therefore, the level of crystallinity and the electric characteristicscan be evaluated by calculating the electron mobility.

The inventors of the present invention sequentially laminated a Ti filmand a Au film on each of the ZnO thin films of the Example and theComparative example by an evaporation method to form an electrode, andconducted the hole measurement so as to measure the electron mobility.

The electron mobility was about 2 cm2/V·sec in the Comparative example,whereas the electron mobility was about 30 cm2/V·sec in the Example.Consequently, it was ascertained that both the crystallinity and theelectric characteristics of the Example were greatly improved ascompared to those of the Comparative example.

As described above, the electronic components according to preferredembodiments of the present invention are preferably used as componentsfor image equipment, and are particularly suitable for use aslight-emitting elements of optical pickups used in image equipment.

While the present invention has been described with respect to preferredembodiments, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than those specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

1. A semiconductor device comprising: a single crystal substrateprimarily including zinc oxide and having a zinc-polar surface and anoxygen-polar surface; and at least one layer of thin film primarilyincluding zinc oxide disposed on the zinc-polar surface; wherein the atleast one layer of thin film includes a multilayer film and themultilayer film defines a light-emitting layer; the multilayer filmincludes an n-type contact layer, an n-type clad layer, an active layer,a p-type clad layer and a p-type contact layer; and the n-type contactlayer is a zinc oxide layer that is in contact with the zinc-polarsurface of the single crystal substrate.
 2. The semiconductor deviceaccording to claim 1, wherein the at least one layer of thin film haszinc-polarity.
 3. The semiconductor device according to claim 1, furthercomprising a transparent electrode disposed on the multilayer film. 4.The semiconductor device according to claim 3, wherein the transparentelectrode is made of Indium Tin Oxide.
 5. A method for manufacturing asemiconductor device, comprising the steps of: determining whether asurface of a single crystal substrate primarily including zinc oxide isa zinc-polar surface or an oxygen-polar surface; and forming at leastone layer of thin film primarily including zinc oxide on the zinc-polarsurface; wherein the at least one layer of thin film includes amultilayer film and the multilayer film defines a light-emitting layer;the multilayer film includes an n-type contact layer, an n-type cladlayer, an active layer, a p-type clad layer and a p-type contact layer;and the n-type contact layer is a zinc oxide layer that is in contactwith the zinc-polar surface of the single crystal substrate.
 6. Themethod for manufacturing a semiconductor device according to claim 5,wherein the thin film has zinc-polarity.
 7. The method for manufacturinga semiconductor device according to claim 5, further comprising thesteps of: providing a sputtering apparatus provided with a plasmageneration chamber and a separate film formation chamber; and performingsputtering treatment using the sputtering apparatus so as to form thethin film.
 8. The method for manufacturing a semiconductor deviceaccording to claim 7, wherein the sputtering treatment is performed by amethod selected from the group consisting of an electron cyclotronresonance plasma sputtering method, an inductively coupled plasmasputtering method, a helicon wave excited plasma sputtering method, anion beam sputtering method, and a cluster beam sputtering method.
 9. Themethod for manufacturing a semiconductor device according to claim 5,wherein the thin film is formed by a method selected from the groupconsisting of a molecular-beam epitaxy method, a metal organic chemicalvapor deposition method, a laser molecular-beam epitaxy method, and alaser abrasion method.